The present invention relates to the preparation of electrolyte materials and structures for use in molten-electrolyte fuel cells. Active electrolyte materials such as the alkali metal carbonates, hydroxides and oxides as well as mixtures of these materials are contemplated. Ceramic support materials such as the alkali metal aluminates are employed to provide inert particles as a substrate, or binder material for retention of the active electrolyte within the fuel cell between the anode and cathode.
Although the present invention may have application in a large variety of fuel cells, e.g., the hydrogen-to-oxygen fuel cell employing alkali metal hydroxide electrolyte, it is particularly suited for preparing electolyte structures such as compacts or pastes for fuel cells using molten carbonate mixtures as electrolyte. Fuel cells employing molten carbonate electrolyte can accept various carbonaceous gases as fuels. For example, methanol and carbon monoxide along with hydrogen have been proposed. One source of a fuel gas is that produced in the gasification of coal. This product gas includes carbon dioxide, carbon monoxide and hydrogen. In such a cell the following reactions can occur. At the anode: EQU H.sub.2 + CO.sub.3.sup.= .fwdarw. CO.sub.2 + H.sub.2 0 + 2e.sup.- EQU CO + CO.sub.3.sup.= .fwdarw. 2CO.sub.2 + 2e.sup.-
At the cathode: EQU 2e.sup.- + CO.sub.2 + 1/2 O.sub.2 + 2e.sup.- .fwdarw. CO.sub.3.sup.=
the carbon dioxide gas required at the cathode can be provided from that produced at or delivered to the anode.
The active electrolyte material is generally provided as a mixture of molten alkali metal carbonates at the cell operating temperature. Considerable reduction in melting temperatures can be obtained by using eutectics and other molten mixtures of the carbonates. Table I lists electrolytes that have been suggested by Janz and Lorenz J. Chem. Eng. Data 6 (3), 321-323 (1961) and Reisman, J. Am. Chem. Soc. 81, 807-811 (1959).
TABLE I ______________________________________ Mole Percent Melting System A B C Point, K) ______________________________________ Li.sub.2 CO.sub.3 (A) 999 Na.sub.2 CO.sub.3 (B) 1131 K.sub.2 CO.sub.3 (C) 1172 LiKCO.sub.3 50.0 50.0 0 777.5 Li.sub.2 CO.sub.3 --K.sub.2 CO.sub.3 42.7 0 57.3 771 Li.sub.2 CO.sub.3 --K.sub.2 CO.sub.3 62.0 0 38.0 740-761 Li.sub.2 CO.sub.3 --Na.sub.2 CO.sub.3 52.0 48.0 0 774 Li.sub.2 CO.sub.3 --Na.sub.2 CO.sub.3 --K.sub.2 CO.sub.3 43.5 31.5 25.0 670 Na.sub.2 CO.sub.3 --K.sub.2 CO.sub.3 0 56 44 983 ______________________________________
Molten alkali carbonate compositions other than eutectics are also contemplated for use in fuel cells that operate at temperatures of about 650.degree.-700.degree. C. (923-973 K).
Molten carbonate fuel cells have been suggested as stacks of repeating elements. Each element contains an anode, a cathode with an electrolyte structure or compact separating the two. Anode structures can include porous, sintered nickel possibly alloyed with chromium or cobalt. Suitable means of current collection and an electrically conductive separator plate between the anode and the next cell in the stack are incorporated. Cathodes of similar structure are contemplated of, for instance, porous nickel oxide prepared within the cell by oxidation of sintered nickel structures. The electrolyte structure disposed between the electrodes includes the active electrolyte material of alkali metal carbonates along with an inert, matrix or substrate material. The alkali metal aluminates, particularly lithium aluminate are currently of interest for use as this inert substrate material. The formation of lithium aluminate is favored relative to sodium or potassium aluminate because of its greater stability.
Structural strength and electrolyte retention are closely related to the shape and size of the aluminate particle. A preferred shape for lithium aluminate particles appears to be that of long rods or fibers. Such fibers combine strength with small interstitial dimensions within the electrolyte structure to hold the liquid electrolyte by surface forces.
One method used in the preparation of electrolyte structures involves reaction of finely divided alumina with alkali metal carbonates at temperatures around 900 K. To obtain complete reaction to the desired rod-shaped particles, it has been found that the repetitive steps of cooling, grinding, blending in more alkali carbonates and reheating to reaction temperatures are required over a number of cycles. This complex and repetitive procedure is followed by pressing at 20 to 60 MPa and 720-770 K. Even with this difficult and complex procedure, one cannot ensure preparation of the desired elongated rods of lithium aluminate. Other morphologies such as square bipyramidal particles of gamma lithium aluminate, platelets of alpha and beta lithium aluminate and clumps of either alpha or beta lithium aluminate are sometimes observed.
Therefore, in view of these problems associated with the prior art methods of preparing electrolyte materials for fuel cells, it is an object of the present invention to provide an uncomplicated method for producing rod-shaped particles of lithium aluminate for use as a matrix and support for active electrolyte material.
It is a further object to provide a method for producing such material at reduced temperatures.
It is also an object to provide flexibility in electrolyte preparation to permit completion after cell assembly.